1. Field of the Invention
The present invention relates to a furnace for heating a quartz preform for the fabrication of an optical fiber, and more particularly, to a furnace for heating a porous glass preform comprising quartz glass soot for the purpose of dehydration, addition of dopants and sintering to produce a highly pure quartz glass preform for the fabrication of an optical fiber.
The heating furnace of the present invention can prevent contamination of the preform with impurities and has good durability.
2. Description of the Related Art
To produce a glass preform for an optical fiber by the VAD method or the OVD method, it is necessary to dehydrate a glass soot preform formed by such method and then to densify and sinter the dehydrated soot preform. In some cases, fluorine, which is a dopant for adjusting a refractive index of the glass, is added in the dehydration step and/or the sintering step, or between the dehydration step and the sintering step. For dehydration, sintering and addition of fluorine, a heating furnace equipped with a muffle tube is used.
The conventional muffle tube is made of alumina (cf. Japanese Patent Publication No. 40096/1982 and U.S. Pat. No. 4,338,111) or quartz glass (cf. Japanese Patent Publication Nos. 58299/1983 and 42136/1983). With the muffle tube made of alumina, impurities such as alkali are liberated from the surface of the muffle tube so that the produced preform tends to be crystallized. Since the muffle tube made of quartz glass includes impurities such as copper or water, on one hand, the produced glass preform provides an optical fiber having increased optical absorbance, on the other hand, the muffle tube itself has unsatisfactory heat resistance.
To overcome the above problems, carbon is proposed as a material of the muffle tube used in the heating furnace (cf. WO88/06145, U.S. Pat. application Ser. No. 07/274,995 filed on Oct. 6, 1988 and EP-A1-0 302 121).
One of the conventional heating furnace is shown in FIG. 1. The heating furnace of this type comprises a cylindrical furnace body 5, and a muffle tube 3 which is inserted through the furnace body. A heater 4 is installed inside the furnace body. The furnace body 5 has an inlet 6 for an inert gas, and the muffle tube 3 has an inlet 7 for an atmospheric gas (e.g. Cl.sub.2, SiF.sub.4, He, etc.). The muffle tube 3 consists of an upper part 34, a middle part 35 and a lower part 36.
When the furnace is used, a porous soot preform 1 is supported in the muffle tube by means of a supporting rod 2 and heated.
The muffle tube disclosed in WO88/06145, U.S. Pat. application Ser. No. 07/274,995 filed on Oct. 6, 1988 and EP-A1-0 302 121 is characterized in that at least the inner layer consists of highly pure carbon. Examples of the disclosed designs of the muffle tube wall are as follows:
1. A silicon carbide or quartz wall having a highly pure carbon coating on the inner surface.
2. A highly pure carbon wall having a silicon carbide coating on the outer surface.
3. A wall consisting of an outer layer of silicon carbide and an inner layer of highly pure carbon.
However, each of these constructions has following drawbacks:
1. In the first design, the carbon coating tends to be peeled off or cracked because of difference of coefficients of thermal expansion between silicon carbide or quartz and highly pure carbon or weak bonding of the carbon coating to the silicon carbide or quartz wall. Since the quartz wall is softened and deformed at a temperature of 1500.degree. C. or higher, it is impossible to maintain the bonding between the quartz wall and the carbon coating. Since the silicon carbide wall is corroded with chlorine gas (Cl.sub.2) at a temperature of 900.degree. C. or higher, the life of muffle tube is greatly shortened by treatment with the chlorine gas when the carbon coating is peeled off or cracked.
2. In the second design, since the highly pure carbon generally has gas permeability, a part of the atmospheric gas in the muffle tube reaches the silicon carbide layer. When the chlorine gas kept at a temperature of 900.degree. C. or higher is used, silicon atoms are removed from the silicon carbide layer to leave a carbon layer. Since carbon layer formed through the removal of silicon atoms from the silicon carbide layer has a smaller density than a usual carbon layer, gasses can easily pass through the layer at high temperatures, whereby the glass preform is contaminated with the impurities present outside the muffle tube.
3. The third design has the same problems as those with the second design. In addition, the silicon carbide layer is not a coated material but made of a sintered material, it becomes brittle when it is corroded with the chlorine gas and its life is considerably shortened.
As explained above, with the muffle tube made of the conventional material, the preform should be heated at a limited temperature in a limited atmosphere. In addition, the muffle tube has short life.